1. Field of the Invention
The present invention relates to an optical fiber transmission line suitable for high-speed transmission with a large capacity over a long distance, an optical cable including the same, and an optical transmission system including the same.
2. Related Background Art
Optical transmission systems transmit signal light including a large capacity of information over a long distance at a high speed by way of optical fiber transmission lines. Various proposals have been made in order to realize further larger capacity and longer distance in such an optical transmission system. For example, the optical transmission system disclosed in literature 1xe2x80x94J. -P. Blondel, et al., xe2x80x9cNetwork Application and System Demonstration of WDM Systems with Very Large Spansxe2x80x9d, OFC""2000, PD31 (2000)xe2x80x94is a wavelength division multiplexing (WDM) transmission system which optically transmits a plurality of channels of signals in a wavelength division multiplexing manner, while comprising an optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) employing an optical fiber having an optical waveguide region doped with Er element as an optical amplifying medium, and a Raman amplifier utilizing a Raman scattering phenomenon. Also, in the optical transmission system disclosed in the above-mentioned literature 1, an optical fiber transmission line comprising an optical fiber having a low loss is laid in a repeating section. At least one of transmitting, repeating, and receiving stations is provided with an EDFA and means for supplying Raman amplification pumping light to the optical fiber transmission line. Such a configuration makes repeating sections longer in the optical transmission system disclosed in literature 1.
The optical transmission systems disclosed in literature 2xe2x80x94T. Naito, et al., xe2x80x9c1 Terabit/s WDM Transmission over 10,000 kmxe2x80x9d, ECOC""98, pp. 24-25 (1988)xe2x80x9dxe2x80x94and literature 3xe2x80x94K. Takashina, et al., xe2x80x9c1 Tbit/s (100 chxc2x710 Gbit/s) WDM Repeaterless Transmission over 200 km with Raman Amplifierxe2x80x9d, OFC""2000, FC8 (2000)xe2x80x94are also WDM transmission systems each comprising an EDFA and a Raman amplifier. In the optical transmission systems disclosed in literatures 2 and 3, the optical fiber transmission line laid in a repeating section is constituted by a positive dispersion optical fiber having a low transmission loss, a large effective area, and a positive chromatic dispersion, and a negative dispersion optical fiber, disposed downstream the positive dispersion optical fiber so as to compensate for the chromatic dispersion in the positive dispersion optical fiber, having a negative chromatic dispersion. At least one of transmitting, repeating, and receiving stations is provided with an EDFA and means for supplying Raman amplification pumping light to the negative dispersion optical fiber. Such a configuration restrains the waveform of signal light from deteriorating due to nonlinear optical phenomena and cumulative chromatic dispersion, whereby a larger capacity in optical transmissions and a longer distance in repeating sections are achieved.
The inventor studied the conventional optical transmission systems and, as a result, has found the following problems. Namely, as compared with long-distance optical transmission systems (e.g., a system connecting continents to each other with a submarine optical cable), medium-range optical transmission systems (e.g., a system connecting the mainland and an island to each other with a submarine optical cable) are required to further elongate their repeating sections. This is due to the fact that, in a medium-range optical transmission system connecting the mainland and an island to each other, a transmitting station, a repeating station, or a receiving station is provided only in the mainland or island, whereas there has been an increasing demand for making a non-repeating section between the mainland and island. However, there is a limit to elongation of repeating sections in each of the optical transmission systems disclosed in the above-mentioned literatures 1 to 3.
In the optical transmission system disclosed in the above-mentioned literature 1, the optical fiber transmission line is constituted by one kind of optical fiber alone, whereby it is preferred that the optical fiber have a high dopant concentration in its core region or a small effective area from the viewpoint of Raman amplification efficiency with respect to signal light in this optical fiber. However, when the dopant concentration is higher, transmission loss becomes greater due to Rayleigh scattering caused by the dopant. Also, when the effective area is smaller, nonlinear optical phenomena are more likely to occur, thereby deteriorating the waveform of signal light, thus failing to transmit signal light having a high power. Hence, there is a limit to elongation of repeating sections in the optical transmission system disclosed in literature 1.
In the optical transmission system disclosed in the above-mentioned literature 2 or 3, the optical fiber transmission line is constituted by a positive dispersion optical fiber and a negative dispersion optical fiber. In general, the negative dispersion optical fiber has a high dopant concentration in its core region, whereby its transmission loss is large due to the Rayleigh scattering caused by the dopant. Hence, there is a limit to elongation of repeating sections in the optical transmission systems disclosed in literatures 2 and 3 as well.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical fiber transmission line comprising a structure which enables repeating sections to become further longer and can yield stable transmission characteristics even when pumping light having a higher power is supplied thereto, an optical cable including the same, and an optical transmission system including the same.
The optical fiber transmission line according to the present invention comprises first and second optical fibers successively disposed along a traveling direction of signal light, and an optical multiplexer for supplying Raman amplification pumping light to one of the first and second optical fibers. The first optical fiber comprises an entrance end for receiving signal light and an exit end for emitting the signal light, whereas the second optical fiber comprises an entrance end optically connected to the exit end of the first optical fiber and an exit end for emitting the signal light. Each of the first and second optical fibers may have a core region doped with a refractive index adjustor such as GeO2, or at least one of the first and second optical fibers may have a core region substantially made of pure silica glass.
In particular, in this optical fiber transmission line, the first optical fiber has, as characteristics at a wavelength of 1550 nm, a first effective area Aeff1 and a first chromatic dispersion D1, and has a first length L1. The second optical fiber has, as characteristics at the wavelength of 1550 nm, a second effective area Aeff2 smaller than the first effective area Aeff1 and a second chromatic dispersion D2 different from the first chromatic dispersion D1, and has a second length L2 different from the first length L1. The optical multiplexer is optically coupled to the entrance end of the first optical fiber so as to supply the Raman amplification pumping light to the first optical fiber together with the signal light, or optically coupled to the exit end of the second optical fiber so as to supply the Raman amplification pumping light to the second optical fiber while transmitting therethrough the signal light emitted from the second optical fiber.
Here, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area Aeff is given by the following expression:       A    eff    =      2    ⁢                            π          ⁡                      (                                          ∫                0                ∞                            ⁢                                                E                  2                                ⁢                r                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  r                                                      )                          2            /              (                              ∫            0            ∞                    ⁢                                    E              4                        ⁢            r            ⁢                          xe2x80x83                        ⁢                          ⅆ              r                                      )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
Recently, as optical transmissions attain a larger capacity, attention has been given to distributed Raman amplification technique in which pumping light is supplied to an optical fiber transmission line, so that the optical fiber transmission line itself becomes an optical amplifying medium. Though a high nonlinearity is required for efficiently carrying out Raman amplification, it also induces nonlinear phenomena (e.g., four-wave mixing, self-phase modulation, and cross-phase modulation) other than Raman amplification, thereby causing signals to deteriorate. Such nonlinear phenomena other than Raman amplification are more likely to occur within optical fibers as the signal light power is higher.
For effectively suppressing the above-mentioned unnecessary nonlinear phenomena other than Raman amplification, it is preferred in the optical fiber transmission line that the ratio of the length of the second optical fiber (L2/(L1+L2)) to the total length of the optical fiber transmission line (L1+L2) be 0.2 or more but 0.7 or less as an appropriate length ratio between the first and second optical fibers optically connected to each other.
Preferably, in the optical fiber transmission line, each of the first chromatic dispersion D1 and second chromatic dispersion D2 ( less than D1) is positive and, specifically, the first chromatic dispersion D1 is greater than 17 ps/nm/km, whereas the second chromatic dispersion is greater than 3 ps/nm/km. When the Raman amplification pumping light is supplied to the second optical fiber, the second effective area Aeff2 is preferably greater than 50 xcexcm2, whereas the first effective area Aeff1 is preferably greater than 90 xcexcm2.
In an optical fiber transmission line such as the one mentioned above, it is preferred that Raman amplification pumping light be supplied to the optical fiber having the higher nonlinearity (one yielding the greater Raman gain) As the power of such Raman amplification pumping light becomes higher, signals are more likely to deteriorate due to double Rayleigh scattering and multiple reflections of signal light at the entrance end of the optical fiber on which the Raman amplification pumping light is incident. Therefore, the optical fiber transmission line according to the present invention may comprise a structure in which an optical fiber having a higher nonlinearity is held between optical fibers having a lower nonlinearity. Specifically, the optical fiber transmission line comprises first to third optical fibers successively disposed along a traveling direction of signal light, and an optical multiplexer for supplying Raman amplification pumping light to at least one of the first and third optical fibers. Each of the first to third optical fibers may have a core region doped with a refractive index adjustor such as GeO2, or at least one of the first to third optical fibers may have a core region substantially made of pure silica glass.
The first optical fiber comprises an entrance end for receiving signal light and an exit end for emitting the signal light. The first optical fiber has, as characteristics at the wavelength of 1550 nm, a first effective area Aeff1 and a first chromatic dispersion D1, and has a first length L1. The second optical fiber comprises an entrance end optically connected to the exit end of the first optical fiber and an exit end for emitting the signal light. The second optical fiber has, as characteristics at the wavelength of 1550 nm, a second effective area Aeff2 smaller than the first effective area Aeff1 and a second chromatic dispersion D2 different from the first chromatic dispersion D1, and has a second length L2 different from the first length L1. Further, the third optical fiber comprises an entrance end optically connected to the exit end of the second optical fiber and an exit end for emitting the signal light. The third optical fiber has, as characteristics at the wavelength of 1550 nm, a third effective area Aeff3 greater than the second effective area Aeff2 and a third chromatic dispersion D3 different from the second chromatic dispersion D2, and has a third length L3 different from the second length L2. The optical multiplexer is optically coupled to the entrance end of the first optical fiber so as to supply the Raman amplification pumping light to the first optical fiber together with the signal light, or optically coupled to the exit end of the third optical fiber so as to supply the Raman amplification pumping light to the third optical fiber while transmitting therethrough the signal light emitted from the third optical fiber.
Preferably, in the optical fiber transmission line having the first to third optical fibers as mentioned above, when the Raman amplification pumping light is supplied to the third optical fiber, it is preferred that the ratio of the length of the third optical fiber (L3/(L1+L2+L3)) with respect to the total length (L1+L2+L3) be 0.1 or more but 0.25 or less. The length L1 of the first optical fiber is preferably equal to or longer than the length L3 of the third optical fiber.
Preferably, in the optical fiber transmission line having the first to third optical fibers as mentioned above, each of the first to third chromatic dispersions D1 to D3 (D1 greater than D2, D2 less than D3) is positive and, specifically, the second chromatic dispersion D2 is greater than 3 ps/nm/km, whereas each of the first and third chromatic dispersions D1, D3 is greater than 17 ps/nm/km. Preferably, the second effective area Aeff2 is greater than 50 xcexcm2. Preferably, one of the first effective area Aeff1 and third effective area Aeff3 is greater than 90 xcexcm2.
In the optical fiber transmission line according to the present invention comprising the structure mentioned above, all of the optical fibers constituting the optical fiber transmission line may have a core region substantially made of pure silica glass. Here, in each of the optical fibers constituting the optical fiber transmission line, the relative refractive index difference of the core region with reference to pure silica glass preferably has a maximum value of xe2x88x920.1% or more but +0.1% or less.
In each of the optical fibers constituting the optical fiber transmission line, the optical fiber having the core region substantially made of pure silica glass preferably has a loss of 0.18 dB/km or less at a wavelength of 1550 nm, whereas the fusion-splicing loss between the optical fibers is preferably 0.2 dB or less.
The optical fiber transmission line according to the present invention may further comprise a structure for ameliorating a nonlinear phenomenon between channels included in the signal light reaching the entrance end of the first optical fiber. Also, it may further comprise an additional optical fiber having, as characteristics at a wavelength of 1550 nm, a negative chromatic dispersion and a polarization mode dispersion (PMD) of 0.2 psxc2x7kmxe2x88x921/2 or less, and an optical multiplexer for supplying Raman amplification pumping light to the additional optical fiber from at least one of entrance and exit ends thereof.
The optical cable according to the present invention includes the optical fiber transmission line comprising the structure mentioned above. The optical transmission system according to the present invention comprises a transmitter for transmitting a plurality of channels of signal light, the optical fiber transmission line having the structure mentioned above, and a receiver for receiving a signal propagated through the optical fiber transmission line.